The present invention relates to backwashable filtering devices which are cleaned by backwashing or backflushing the fluid feed through the filter. The present invention is particularly useful in the in-line type of backwashable filtering device such as described in my prior patent application Ser. No. 74,525 filed Sept. 11, 1979, and Ser. No. 92,583 filed Nov. 8, 1979, and also in my prior U.S. Pat. Nos. 4,042,504 and 4,045,345, and is therefore described below with respect to this type of filtering device.
The above-cited patents and applications both relate to backwashable filtering devices including a housing having an inlet connectable to an upstream fluid pipe and an outlet connectable to a downstream fluid pipe, a filter body having an upstream surface communicating with the housing inlet to intercept dirt particles in the fluid flowing in the foreward direction from the housing inlet to the housing outlet, a backwash nozzle within the housing and having a nozzle inlet disposed adjacent to the upstream surface of the filter body, means for effecting relative movement between the filter body and the backwash nozzle, and means for connecting the backwash nozzle to the atmosphere for backwashing the filter body. This type of backwashable filtering device operates quite well when the line pressure is relatively high, e.g. over about 1.5 or 2 atmospheres. There are many systems, however, where the line pressure is relatively low such that when the backwash nozzle is connected to the atmosphere for backwashing the filter body, the outlet pressure may be insufficient to effect the backwashing of the filter body.